The present invention relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fine pore enthalpy exchange barrier for a fuel cell power plant that exchanges heat and water exiting the plant back into the plant to enhance water balance and energy efficiency of the plant.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles, or on-site generators for buildings. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPENxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, nonconductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
In operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode including water resulting from proton drag through the PEM electrolyte and rates at which water is removed from the cathode and at which water is supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode, the PEM may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance.
As fuel cells have been integrated into power plants developed to power transportation vehicles such as automobiles, trucks, buses, etc., maintaining a water balance within the power plant has become a greater challenge because of a variety of factors. For example, with a stationary fuel cell power plant, water lost from the plant may be replaced by water supplied to the plant from off-plant sources. With a transportation vehicle, however, to minimize fuel cell power plant weight and space requirements, the plant must be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset water losses from gaseous streams of reactant fluids passing through the plant. For example, any water exiting the plant through a cathode exhaust stream of gaseous oxidant or through an anode exhaust stream of gaseous reducing fluid must be balanced by water produced electrochemically at the cathode and retained within the plant.
An additional requirement for maintaining water self-sufficiency in fuel cell power plants is associated with components necessary to process hydrocarbon fuels, such as methane, natural gas, gasoline, methanol, diesel fuel, etc., into an appropriate reducing fluid that provides a hydrogen rich fluid to the anode electrode. Such fuel processing components of a fuel cell power plant typically include a boiler that generates steam; a steam duct into which the hydrocarbon fuel is injected; and an autothermal reformer that receives the steam and fuel mixture along with a small amount of a process oxidant such as air and transforms the mixture into a hydrogen-enriched reducing fluid appropriate for delivery to the anode electrode of the fuel cell. The fuel processing components or system water and energy requirements are part of an overall water balance and energy requirement of the fuel cell power plant. Water made into steam in the boiler must be replaced by water recovered from the plant such as by condensing heat exchangers in the cathode exhaust stream and associated piping.
A common approach to enhancing water recovery and retention is use of condensing heat exchangers in exhaust streams of the power plant wherein the exhaust streams are cooled to a temperature at or below their dew points to precipitate liquid water from the exhaust streams so that the liquid may be returned to the power plant. An example of a PEM fuel cell power plant using a condensing heat exchanger is shown in U.S. Pat. No. 5,573,866 that issued on Nov. 12, 1996 to Van Dine et al., and is assigned to the assignee of the present invention, and which patent is hereby incorporated herein by reference. Many other fuel cell power plants that use one or more condensing heat exchangers are well-known in the art, and they typically use ambient air streams as a cooling fluid passing through the exchanger to cool the plant exhaust streams. In Van Dine et al., the heat exchanger is used to cool a cathode exhaust stream, which upon leaving a cathode chamber includes evaporated product water and some portion of methanol, the reducing fluid, that has passed through the PEM. The condensing heat exchanger passes the cathode exhaust stream in heat exchange relationship with a stream of cooling ambient air, and then directs condensed methanol and water indirectly through a piping system back to an anode side of the cell.
While condensing heat exchangers have enhanced the water recovery and energy efficiency of fuel cell power plants, the heat exchangers encounter decreasing water recovery efficiency as ambient temperatures increase. Where the power plant is to power a transportation vehicle such as an automobile, the plant will be exposed to an extremely wide range of ambient temperatures. For example where an ambient air coolant stream passes through a heat exchanger, performance of the exchanger will vary as a direct function of the temperature of the ambient air because decreasing amounts of liquid precipitate out of power plant exhaust streams as the ambient air temperature increases.
An additional requirement of using such condensing heat exchangers in fuel cell power plants powering transportation vehicles is related to operation of the vehicles in temperatures below the freezing temperature of water. Because water from such exchangers is often reintroduced into the PEM fuel cells of the plant, the water may not be mixed with conventional antifreeze to lower its freezing temperature. Propylene glycol and similar antifreezes would be adsorbed by the catalysts in the cells decreasing cell efficiency, as is well known.
Accordingly, known fuel cell power plants that employ ambient air as the cathode oxidant and/or that use condensing heat exchangers are incapable of efficiently maintaining a self-sufficient water balance when operating at high ambient temperatures because of their above described characteristics. It is therefore highly desirable to produce a fuel cell power plant that can achieve a self-sufficient water balance without a condensing heat exchanger while minimizing plant operating energy requirements.
A fine pore enthalpy exchange barrier is disclosed for use with a fuel cell power plant. The barrier includes a support matrix that defines pores and a liquid transfer medium that fills the pores creating a gas barrier. An inlet surface of the fine pore enthalpy exchange barrier is positioned in contact with a process oxidant inlet stream entering a fuel cell power plant, and an opposed exhaust surface of the barrier is positioned in contact with an exhaust stream exiting the plant so that water and heat exchange from the exhaust stream directly into the process oxidant inlet stream. The support matrix defines pores having a pore-size range of about 0.1-100 microns; the matrix is hydrophilic so that it is capable of being wetted by the liquid transfer medium resulting in a bubble pressure that is greater than 0.2 pounds per square inch (xe2x80x9cp.s.i.xe2x80x9d); and, the matrix is chemically stable in the presence of the liquid transfer medium.
A first exemplary group of support matrixes includes rigid support matrixes, such as: rigid, porous, graphite layers; rigid, porous, graphite-polymer layers; rigid, inorganic-fiber thermoset polymer layers; glass fiber layers; synthetic-fiber filter papers treated to be wettable; porous metal layers; perforated metal layers wherein such perforations may include particulate matter secured within the perforations defining an acceptable fine pore-size range; and a plurality of differing layers of those support matrixes. A second exemplary group of support matrixes includes flexible support matrixes, such as: inorganic fiber layers, papers or felts with or without compatible polymer binders; natural fiber layers, papers or felts with or without compatible polymer binders; organic fiber layers, papers or felts with or without compatible polymer binders; porous compatible plastics with or without wettability treatments; mixtures of carbon blacks and compatible polymer binders with or without reinforcing glass fibers; and, a plurality of differing layers of these flexible support matrixes with or without compatible binders. By use of the word xe2x80x9ccompatiblexe2x80x9d, it is meant that the above listed materials are chemically compatible with the liquid transfer medium.
To provide support for the flexible support matrixes, mesh layers may be positioned adjacent the opposed inlet and exhaust surfaces of the fine pore enthalpy exchange barrier, along with plastic flow guides adjacent the mesh layers to support the mesh layers and to facilitate flow of the oxidant inlet stream and plant exhaust stream into contact respectively with the inlet and exhaust surfaces of the enthalpy exchange barrier. Some of the flexible support matrixes may include only the plastic flow guides positioned adjacent the inlet and exhaust surfaces of the enthalpy exchanger barriers.
Preferably the support matrix has a high thermal conductivity. This helps transfer heat axially from the exhaust stream to the process oxidant inlet stream of ambient air to thereby minimize freezing of an oxidant inlet when operating at very low ambient temperatures. The liquid transfer medium may include water, aqueous salt solutions, aqueous acid solutions, and organic antifreeze water solutions, wherein the transfer medium is capable of sorbing a fluid substance consisting of polar molecules such as water from a fluid stream consisting of polar and non-polar molecules. The fine pore enthalpy exchange barrier may be disposed within a structure of a direct mass and heat transfer device in fluid communication with process oxidant inlet and plant exhaust streams so that the structure and barrier cooperate to restrict bulk mixing of the inlet and exhaust streams. The structure may define manifolds, passageways, and seals to direct the inlet and exhaust streams through the device and into contact with the opposed inlet and exhaust surfaces of the fine pore enthalpy exchange barrier.
In another embodiment, the fine pore enthalpy exchange barrier includes a support matrix having a multi-layer, dual pore-size configuration, wherein a central layer is surrounded by opposed exterior layers and the exterior layers define pores having a larger pore-size range than pores defined by the central layer; the central layer defines less than 25 per cent (hereafter xe2x80x9c%xe2x80x9d) of the total void volume of the support matrix; and the matrix is filled to greater than 35% of its total void volume with a liquid transfer medium so that the central layer is saturated with the transfer medium. The central layer thereby provides a gas barrier between the inlet and opposed exhaust surfaces of the support matrix. In the event of changed operating conditions, the liquid transfer medium may therefore move between the central layer and the exterior layers without having to move out of the fine pore enthalpy exchange barrier into the inlet oxidant stream or exhaust stream. By using a transfer medium that is a mixture of a non-volatile compound and water at operating conditions of the mass and heat transfer device in that embodiment, heated water within the exhaust stream may transfer directly into the inlet stream without loss of the liquid transfer medium from the support matrix as operating conditions change.
An additional embodiment may include a transfer medium circulating loop, wherein the transfer medium is circulated through the support matrix, and replenished when necessary, to further support maintenance of a gas barrier by the liquid transfer medium within the support matrix so that the exhaust stream does not mix directly with the inlet stream.
In operation of a fuel cell power plant using a fine pore enthalpy exchange barrier, as heated water vapor generated within the fuel cell moves from the plant exhaust stream directly through the fine pore enthalpy exchange barrier to humidify the inlet stream, sensible and latent heat also exchange between the inlet and exhaust streams, cooling the exhaust stream and heating the inlet oxidant stream directly with heat from the water within the exhaust stream. Evaporation of the exchanging water at the inlet surface of the fine pore enthalpy exchange barrier into the oxidant inlet stream also results in cooling of the inlet surface of the barrier, thereby increasing a temperature differential between the inlet and exhaust surface. That in turn results in increased rates of heat and mass transfer from the exhaust stream into the inlet stream. A dry oxidant inlet stream, resulting for example from operation of the fuel cell power plant in a dry climate, will thus result in more rapid evaporation of water from the barrier into the oxidant inlet stream. Therefore the fine pore enthalpy exchange barrier automatically increases humidification and heating of the oxidant inlet stream as the stream becomes drier. Additionally, by using a low volatility liquid transfer medium such as a salt solution having a substantial freezing point depression or by use of an antifreeze water solution, the fine pore enthalpy exchange barrier facilitates efficient transfer of water and heat from the plant exhaust stream into the oxidant inlet stream at a wide range of temperatures, without need for pre-heating the mass and heat transfer device housing the barrier; and also protects the enthalpy exchange device from mechanical damage due to freezing of water.
Accordingly, it is a general object of the present invention to provide a fine pore enthalpy exchange barrier for a fuel cell power plant that overcomes deficiencies of prior art fuel cell power plants.
It is a more specific object to provide a fine pore enthalpy exchange barrier for a fuel cell power plant that transfers heat and water vapor from a plant exhaust stream directly into a plant inlet stream.
It is yet another object to provide a fine pore enthalpy exchange barrier for a fuel cell power plant that enhances a water balance and decreases volume and weight of the plant without utilizing a condensing heat exchanger.
It is still a further object to provide a fine pore enthalpy exchange barrier for use within a mass and heat transfer device of a fuel cell power plant that provides a liquid barrier to gas movement between oxidant inlet and plant exhaust streams passing opposed inlet and exhaust surface of the barrier.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.